It is indeed skirmish only, unfortunately, because this relies on a few mechanics that only work in skirmish it won't function in GC at all, we'll have to come up with another way for you to customize your fleet.

I read in an earlier post that you guys wouldn't be adding the Infinity, but perhaps you could add it in GC and only one could be constructed at a time, similar to the death star or super star destroyer in the vanilla game. Maybe you could balance it out by giving the covenant the ability to construct a Super assault carrier (like the one in halo reach that was destroyed by the improvised slipspace bomb), which is nearly twice as large as the Infinity. I'd be kind of disappointed if the Infinity didn't make it in but either way this is a great mod.

Actually it was nearly completed (for emergency purposes) by the end of the war but they decided to keep it a secret so that they could upgrade it with all the new technology they were gathering i.e. Covenant and Forerunner tech.